The present invention relates to a liquid crystal display device provided with a construction for protecting switching elements, which are formed over an active matrix substrate, from static electricity.
Liquid crystal display devices are widely used for the display of various kinds of images including still pictures or moving pictures.
Liquid crystal display devices are classified into two types. There is a first type in which a layer of liquid crystal compounds (a liquid crystal layer) is clamped between two insulating substrates, at least one of which is made of transparent glass or the like, and predetermined pixels are turned on or off by selectively applying a voltage to various kinds of pixel-forming electrodes formed over the insulating substrates; and there is a second type in which active elements made of the aforesaid various kinds of electrodes and switching elements, such as pixel-selecting thin-film transistors (TFTs), are formed, and predetermined pixels are turned on or off by selectively driving these active elements.
The latter type of liquid crystal display device is called an active matrix type device and has become a leading liquid crystal display device because of its contrast performance, its high-speed display performance and the like.
In general, the active matrix type of liquid crystal display device is of a so-called vertical electric field type in which an electric field for changing the direction of alignment of a liquid crystal layer is applied between a plurality of scanning electrodes, a plurality of video signal electrodes and pixel electrodes, all of which are formed over an active matrix substrate, which form one insulating substrate, and a common electrode which is disposed over a color filter substrate, which is forms the other substrate. In addition, in the periphery of the effective display area of the active matrix substrate of the active matrix type of liquid crystal display device, a protecting circuit for protecting thin-film transistors or other electrodes from static electricity, which enters from outside, is formed on the scanning signal lines (in this case, gate signal lines) for driving the thin-film transistors or the like, which form the switching elements, and the video signal lines (in this case, drain signal lines).
FIG. 10 is a top plan view illustrating an essential electrode pattern of an active matrix substrate of the type used in a conventional active matrix type of liquid crystal device. In FIG. 10, symbol SUB1 denotes an active matrix substrate, symbol SUB2 denotes a color filter substrate (in FIG. 10, there is shown only an external line which represents the bonding position of the color filter substrate), symbol COM1 denotes common electrode connecting terminals, symbol SL denotes a sealing material which bonds and secures the active matrix substrate SUB1 and the color filter substrate SUB2 to each other, symbol DL denotes video signal lines (hereinafter referred to as the drain lines), and symbol CHI denotes driving circuits (integrated circuits).
FIG. 11 is a side view illustrating the structure of an essential portion of a side surface C of a top side of the substitution FIG. 10. In FIG. 11, symbols identical to those used in FIG. 10 correspond to portions identical to those shown in FIG. 10, and symbol GI denotes an insulation layer and symbol PSV1 denotes a transparent protective layer. Both the insulation layer GI and the transparent protective layer PSV1 are made of an insulation material such as SiN. Although an alignment layer is formed over the top surface of the transparent protective layer PSV1 before the transparent protective layer PSVI is applied to the color filter substrate, the illustration of the alignment layer is omitted in FIG. 11. Scanning signal lines (gate lines) which extend sideways as viewed in FIG. 10 to cross the drain lines DL, and driving circuits (integrated circuits) for applying scanning driving signals to the gate electrodes are formed over the active matrix substrate SUB1, but the illustration of such gate tines and driving circuits is omitted in FIG. 10.
The active matrix substrate SUB1, in its manufacturing process, is worked into a shape having a panel size by forming individual signal lines, thin-film transistors, a short bar, a short-circuit detecting terminal and the like over a mother glass and cutting the mother glass inside the short bar and the detecting terminal after the application of the required deposition. Accordingly, the cut ends of the drain tines DL are exposed on the side surface of the portion C of FIG. 10, as shown in FIG. 11. The drain lines DL are covered with the protective layer PSV1 over the surface of the active matrix substrate SUB1, but the cut ends of the drain lines DL are exposed on the side surface of a cut edge of the active matrix substrate SUB1.
Since the cut ends of the drain lines DL are exposed on the side surface of the active matrix substrate SUBI, as described above, there is the problem that when static electricity occurs in the active matrix substrate or in the vicinity of the outside thereof in handling the substrate during the manufacturing process, this static electricity enters the drain lines DL through the cut ends thereof and breaks down switching elements, such as thin-film transistors connected to the drain lines, or disconnects other lines. If a thin-film transistor breaks down or a disconnection occurs in a line, a so-called line defect occurs in the screen of a finished liquid crystal display device, degrading the image quality thereof to a remarkable extent.
An object of the present invention is to provide a liquid crystal display device which prevents breakdown of switching elements, such as thin-film transistors, or the disconnection of other lines by effectively absorbing static electricity which enters drain lines, thereby enabling production of a high-quality image display device.
To achieve the above object, the present invention is characterized by a construction in which conductive layers are formed above drain lines and are connected to common electrode connecting terminals of a color filter substrate so that static electricity which enters drain lines is effectively absorbed. A typical example of such a construction is described below.
In a liquid crystal display device in which an active matrix substrate has a plurality of switching elements for selecting pixels formed in a display area, a plurality of scanning signal lines for applying driving signals to the switching elements, a plurality of video signal lines, pixel electrodes and common electrode connecting terminals, the active matrix substrate is bonded to a color filter substrate having common electrodes which constitute the pixels, with a liquid crystal material interposed between both substrates. The common electrode connecting terminals and the common electrodes are connected to each other via a conductive paste, and the periphery of the display area is secured with a seating material. The liquid crystal display device is provided with a plurality of conductive layers which are formed over video signal lines whose cut ends are exposed on a side surface of the active matrix substrate, an insulation layer interposed between the conductive layers and the video signal lines, and a connecting line layer which connects the conductive layers to the common electrode connecting terminals.
In addition, each of the conductive layers is formed in a linear shape which is parallel to a longitudinal direction of the video signal lines, at a position corresponding to a portion between adjacent ones of the plurality of video signal lines, whereby it is possible to effectively absorb static electricity which enters the drain lines.
In the liquid crystal display device having the above-described construction, the electrostatic breakdown of switching elements or other electrodes in the manufacturing process is decreased to a remarkable extent, whereby a high-quality image display device can be obtained and the manufacturing yield factor is improved to a great extent.
As a matter of course, the present invention is not limited to only the above-described construction and various modifications can be made without departing from the technical concept of the present invention. For example, although the above-described construction takes account of the entrance of static electricity through the cut ends of the drain lines, similar conductor layers may also be formed near the cut ends of gate lines Which cross the drain lines so that static electricity is also prevented from entering the gate lines.